Fluid supply systems use motors to drive pumps and transfer fluids from supply reservoirs, such as wells, to demand reservoirs, such as tanks. A sensor measures a characteristic of the fluid, and a controller controls operation of the motor. In some systems, the controller measures a level of the fluid in a tank and controls operation of the motor to maintain the level within a range. When the level reaches the low end of the range, the controller turns the motor on and keeps it on until the level reaches the high end of the range. In other systems, the speed of the motor is controlled to maintain a fluid characteristic within predetermined parameters. A variable speed loop controls the speed of the motor within a variable speed range to gradually increase or decrease the pumping rate and thereby reduce the variability of the fluid characteristic. Induction motors are frequently used in fluid supply systems.
Induction motors are very efficient at rated torque and speed but are inefficient at light loads. Efficiency losses can be due to, among others, core losses including hysteresis and eddy-current losses which depend on flux density and voltage frequency, copper losses which depend from current levels, and friction. Imbalances among these sources of losses, particularly between copper and core losses, result in reduced efficiency at light loads.
Many single phase systems currently in operation were not designed to operate at a variable speed. In many instances, variable speed control of a single phase motor magnifies the perceived acoustic noise generated by torque pulsations. While it is desirable to provide a variable speed system capable of operating with a variety of motors in new and retrofit applications, it is also desirable to reduce or eliminate torque pulsations and to efficiently operate the motors.